1. Field of the Invention
The invention relates to a circulation type document feeder which is used for a copy machine equipped with a document stacker on which document sheets are stacked, carried from the stacker sheet by sheet to be exposed on a platen glass, and returned to the document stacker after exposure.
2. Description of the Prior Art
A circulation type document feeder generally makes one copy at one circulation of a document, so the document must be circulated the same number of times as copies needed. In this type of device, document sheet separation efficiency of a document feed device is very important. The conventional bottom feed-upper return method is known to be excellent in document sheet separation efficiency. By this method, document sheets are stacked on a stacker and the bottom document sheet of the stack is separated from the stack and conveyed to the following unit sheet by sheet. After being exposed, the document sheet is returned to the top of the document stack. This operation is repeated.
The bottom sheet is fed by two methods. One is separating a document sheet by absorption using air suction and the other is separating by friction using a document sheet feed belt and a roller which comes into contact with the belt. The latter compares favorably with the former in that the cost is low, the size is compact and its maintenance and adjustments are easy. A typical example of this type of document feeder is shown in FIG. 12.
As shown in FIG. 12, the document feeder 20 is sharply inclined forwards so that the leading edge of the document sheet to be fed is set downward. As a result, the document stacker 21 is inclined and the document feed opening 22 is set at the lowest part of the stacker 21. A document sheet `D` which is delivered from the first document feed section 23 close to the document feed opening 22 of the stacker 21 is guided from the second document feed section 24 to the platen glass 11 of the copy machine 10 through the document sheet path 25. The document `D`, is moved by the carrier belt 26 onto the platen glass 11 in the right place for exposure and the optical exposure system 12 is operated. After that, the document sheet is moved backward by the carrier belt 26 and returned onto the document stacker 21 from the outlet 28 installed above the document feed opening 22 through return path 28. In this way, the document sheet which has been processed is returned to the top of the unprocessed document sheets on the stacker 21. In this case, processed documents and unprocessed documents are divided by a partition board 29 which is installed close to the document feed opening 22. This partition board 29 is turned counterclockwise after the last unprocessed document is processed and stacked again. As a result, the partition board 29 is on the top of the stack.
The conventional document feed method is explained above. The problems of the conventional method are as follows.
In the conventional method, the document sheet stacker must be set at a steep angle. In other words, it is necessary that the inclination of the stacker 21 is such that the height of the machine is very great. The second stacking is conducted by gravity using the weight of a sheet of paper itself. As a result, the leading edges of document sheets are not arranged well. Furthermore, a document sheet is stopped sometimes because of folds or curls in the paper. Document paper separation efficiency may deteriorate a great deal because of those problems mentioned above. Inclination of the document sheet stacker 21 is high, so there is the possibility that the second stacking document paper may go under the partition board 29. Furthermore, the document must be returned to the document stack after being exposed at the correct position on the platen glass, so a great deal of time is needed to exchange document sheets. Generally speaking, document sheets which have been processed and stacked again are slightly curled on both sides. This type of document sheet causes many problems when it is fed to the document sheet feed opening. Sometimes it causes no feed, double feed or folding of the document sheet.